A. A'Campo-Neuen and J. Hausen gave an example of an algebraic torus action on an open subset of the affine four space that admits no quotient in the category of algebraic varieties. We show that this example admits a quotient in the category of dense constructible subsets and thereby answer a question of A. Białynicki-Birula.

The aim of this paper is to construct open sets with good quotients by an action of a reductive group starting with a given family of sets with good quotients. In particular, in the case of a smooth projective variety X with Pic(X) = 𝒵, we show that all open sets with good quotients that embed in a toric variety can be obtained from the family of open sets with projective good quotients. Our method applies in particular to the case of Grassmannians.

This is the second of a series of papers dealing with an analog in Arakelov geometry of
the holomorphic Lefschetz fixed point formula. We use the main result of the first paper
to prove a residue formula "à la Bott" for arithmetic characteristic classes living on
arithmetic varieties acted upon by a diagonalisable torus; recent results of Bismut-
Goette on the equivariant (Ray-Singer) analytic torsion play a key role in the proof.

We compute the essential dimension of the functors Formsn,d and Hypersurfn,d of equivalence classes of homogeneous polynomials in n variables and hypersurfaces in ℙn−1, respectively, over any base field k of characteristic 0. Here two polynomials (or hypersurfaces) over K are considered equivalent if they are related by a linear change of coordinates with coefficients in K. Our proof is based on a new Genericity Theorem for algebraic stacks, which is of independent interest. As another application of the...

2000 Mathematics Subject Classification: Primary 20F55, 13F20; Secondary 14L30.We construct invariant polynomials for the reflection groups
[3, 4, 3] and [3, 3, 5] by using some special sets of lines on the quadric P1 × P1
in P3. Then we give a simple proof of the well known fact that the ring of
invariants are rationally generated in degree 2,6,8,12 and 2,12,20,30.

Let the special linear group G : = SL₂ act regularly on a ℚ-factorial variety X. Consider a maximal torus T ⊂ G and its normalizer N ⊂ G. We prove: If U ⊂ X is a maximal open N-invariant subset admitting a good quotient U → U ⃫N with a divisorial quotient space, then the intersection W(U) of all translates g · U is open in X and admits a good quotient W(U) → W(U) ⃫G with a divisorial quotient space. Conversely, we show that every maximal open G-invariant subset W ⊂ X admitting a good quotient W...

The purpose of this article is to give, for any (commutative) ring A, an explicit minimal set of generators for the ring of multisymmetric functions TSAd(A[x1,⋯,xr])=A[x1,⋯,xr]⊗Ad𝔖d as an A-algebra. In characteristic zero, i.e. when A is a ℚ-algebra, a minimal set of generators has been known since the 19th century. A rather small generating set in the general case has also recently been given by Vaccarino but it is not minimal in general. We also give a sharp degree bound on the generators, improving the degree bound previously...

The main purpose of this article is to give an explicit algebraic action of the group
S3 of permutations of 3 elements on affine four-dimensional complex space which is not
conjugate to a linear action.

A concept of a slice of a semisimple derivation is introduced. Moreover, it is shown that a semisimple derivation d of a finitely generated commutative algebra A over an algebraically closed field of characteristic 0 is nothing other than an algebraic action of a torus on Max(A), and, using this, that in some cases the derivation d is linearizable or admits a maximal invariant ideal.